This article was originally published in the March/April 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

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Home Energy Magazine Online March/April 1996

Mechanical Ventilation for the Home

by Don Stevens

Don Stevens is a ventilation consultant in Keyport, Washington.

A ventilation system gives occupants control over a home's air change rate and thermal comfort. This article discusses the options available today for good mechanical ventilation.

All houses and apartments need an efficient way to exhaust stale, moist indoor air and introduce outdoor air. Good ventilation makes a house healthier for occupants and protects the building structure from moisture damage (see Fundamentals of Moisture in Houses, HE Nov/Dec '95, p.11).

Traditionally, home builders have relied on natural ventilation and infiltration provided by random gaps and cracks in the building shell. The occupants can control ventilation in these houses only by operating the windows and doors. Natural ventilation depends on wind speed and temperature differences between indoors and outdoors. A house that breathes on its own often has too much ventilation in the winter, resulting in high heating bills and uncomfortable drafts. On mild or calm days in other seasons, the house often has too little ventilation.

Houses built today are tighter than houses of 50 years ago, while the number of pollutants in the home has grown (although smoking is less prevalent). Choosing less toxic building materials at the design stage can limit pollutants in a new home, but this option isn't available once the home is built. Fireplaces, wood stoves, and other combustion appliances increase pollutant levels. Moisture, pets, furnishings, cigarette smoke, aerosol sprays, household cleaners, and hobby supplies add to the mix.

A well-designed mechanical ventilation system with good controls is important to provide consistent ventilation through all seasons of the year for all housing. Fans exhaust stale air. Leaks, passive inlets, or another fan draws in outdoor air. Maintaining good indoor air quality in a house requires reducing pollution sources and providing adequate ventilation to remove and dilute pollutants.

In warm climates, good indoor air quality may be achieved with supply ventilation, but most systems described here deal with exhaust ventilation for temperate and cold climates.

Ventilation Codes
Code requirements for ventilation systems vary in different jurisdictions and utility programs. The local building department is the best source of information on specific code requirements for an area. This article addresses good practices that meet the ventilation standard set by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).

Most building and ventilation codes and utility programs in the United States are based on ASHRAE 62. (ASHRAE develops guidelines and standards for engineering design, building practice, and building codes throughout the United States.) ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, requires mechanical spot ventilation for specific sources of indoor pollutants. It also requires general ventilation that operates whenever the house is occupied to maintain indoor air quality. The number of occupants determines the amount of general ventilation required. Let's look at these two types of ventilation.

Types of Ventilation
Spot ventilation removes moisture, odors, and pollutants directly at the source. ASHRAE requires a ventilation rate of at least 50 cubic feet per minute (CFM) for a bath fan. If the fan is operated continuously, a reduction to 20 CFM is allowed. A kitchen ventilation fan over or near the stove with a minimum flow of 100 CFM is required for removing cooking fumes; a 25-CFM pickup is allowed if the fan is operated continuously. Spot ventilation fans must be vented to the outside.

General indoor air quality (IAQ) ventilation removes contaminants and delivers outdoor air to the occupied rooms of a house. The current ASHRAE standard recommends a minimum of 0.35 air changes per hour (ACH)-that is, in one hour 35% of the home's air is exhausted and replaced with outdoor air. A proposed revision to the ASHRAE 62 standard uses 5 CFM per 100 ft2 of household area as a simpler calculation method. In either case, the ventilation rate must be not less than 15 CFM for each occupant, assuming two occupants for the master bedroom and one for each additional bedroom. I recommend that the ventilation installer use 0.5-0.6 ACH for an upper limit on ventilation rate.

The ASHRAE standard does not require mechanical ventilation, but we know from blower-door and tracer-gas testing that only mechanical systems provide outdoor air reliably through all seasons.

Ventilation Approaches
Most builders use one of four approaches to provide spot and general IAQ ventilation (see Table 1 for a summary of advantages and disadvantages of each approach). Notice that none of these systems relies explicitly on cracks and gaps in the house for outdoor air. Each ventilation system includes an exhaust fan and a way to bring outdoor air into the house either passively or mechanically.

Double-Duty Fan
A single bath or laundry room fan, centrally located, acts both as a spot ventilator for the bathroom or laundry room and as the general IAQ ventilation fan. A switch, interval timer, humidity control, or occupancy sensor controls the fan for spot ventilation. A 24-hour clock timer wired to the fan provides automatic general ventilation. Occupants schedule ventilation times. Both individual controls and integrated sensors are available from American ALDES, Broan, Grasslin, Intermatic, NuTone, Tamarack Technologies, and Tork (see Table 2). These fans may sense humidity, carbon dioxide, cigarette smoke, or motion.

The fan should be quiet, with a sound rating of 1.5 sones or less. Most bath fans are rated at 3 to 4 sones and are too noisy for the extended operation needed for a general IAQ ventilation fan (see Table 3 for a list of quiet, low energy fans). Panasonic has set the quiet fan standard, with several 0.5- and 1.0-sone fans. Other manufacturers are introducing new fans to try to catch up.

Table 2. Fan, HRV, and Control Manufacturers

Manufacturer

Phone Number

Products Available

American ALDES Ventilation Corporation

Tel:(800)255-7749

CM, DH, RS, T&C, WP

Broan Manufacturing Company

Tel:(800)558-1711

CM, HRV, RS, RM, T&C, WP

Carrier Corporation

Tel:(800)227-7437

HRV

Continental Fan

Tel:(800)320-0504

RS

Crispaire Corp/E-Tech division

Tel:(404)458-6643

VHP

DEC International/Therma-Stor Products Group

Tel:(800)533-7533

DH, RS, RM, T&C, VHP, WP

Duro Dyne Corporation/DuroZone division

Tel:(800)899-3876

T&C

Fan America

Tel:(800)838-4074

CM, RM

Fantech

Tel:(800)747-1762

RS, RM

Honeywell

Tel:(800)328-5111

HRV, T&C

Grasslin Controls Corporation

Tel:(914)664-3542

T&C

Intermatic Incorporated

Tel:(815)675-2321

T&C

NuTone Incorporated

Tel:(800)543-8687

CM, T&C

Panasonic

Tel:(201)348-7231

CM, T&C

RAYDOT Incorporated

Tel:(800)328-3813

HRV

Stirling Technology Incorporated

Tel:(800)535-3448

HRV

Tamarack Technologies Incorporated

Tel:(800)222-5932

CM, HRV, T&C

Tork

Tel:(201)825-9696

T&C

Venmar/VanEE

Tel:(800)667-3717

HRV

CM

Ceiling-mounted fans

RM

Remote-mounted multiport fans

DH

Dehumidifying ventilator

T&C

Timers and controls

HRV

Heat recovery ventilator

VHP

Ventilating heat pump water heater

RS

Remote-mounted single-port fans

WP

Wall inlet ports

Table 3. Quiet Fans Suitable For General IAQ Ventilation

Ceiling-Mounted Fans

CFM @ 0.1wg

CFM @ 0.25wg

Fan Input Wattage

HVI Sound Rating

Fan Efficiency

American ALDES CMV80

80

55

32W

0.8 sones

.03-.05

American ALDES CMV100

95

75

34W

1.2 sones

.03-.06

American ALDES CMV125

120

105

40W

1.5 sones

.04-.08

Broan S90

90

75

50W

1.5 sones

.02-.04

Fan America SMV80

80

60

35W

0.8 sones

.03-.05

Fan America SMV100

100

82

37W

1.2 sones

.03-.06

Fan America SMV140

120

110

40W

1.5 sones

.04-.08

NuTone QT80

80

63

47W

1.5 sones

.02-.04

NuTone QT90

90

85

50W

1.5 sones

.02-.05

NuTone QT100L

100

80

30W

1.5 sones

.04-.08

NuTone LS50

50

23

16W

0.3 sones

.04

NuTone LS80

80

55

24W

0.8 sones

.04-.07

NuTone LS100

100

80

33W

1.5 sones

.04-.07

Panasonic 05VQ

50

31

12W

0.5 sones

.05-.08

Panasonic 07VQ

70

52

15W

0.5 sones

.05-.10

Panasonic 08VQ

90

70

17W

1.0 sones

.06-.12

Panasonic 11VQ

110

88

19W

1.5 sones

.07-.14

Panasonic 20VQ

190

130

31W

1.5 sones

.07-.12

Remote Single-Point Inline Fans

CFM @ 0.4wg

Fan Input Wattage

Fan Efficiency

American ALDES SPV200

75-230

125W

.03-.22

Broan SP100

97

50W

.09

Broan SP140

141

85W

.08

Continental AXC100A

85

35W

.11

Continental AXC125A

80

40W

.09

Fantech F/FR100

90

70W

.06

Fantech F/FR125

110

70W

.07

Rosenburg R100 (DEC)

83

50W

.08

Remote Multipoint Fans

CFM @ 0.4wg

Fan Input Wattage

Fan Efficiency

American ALDES VMP-S

65-130

75W

.04-.08

American ALDES VMP-K

100-180

80W

.06-.11

American ALDES MPV200

75-230

90W

.04-.12

American ALDES MPV300

200-330

120W

.08-.13

Broan MP100

98

50W

.09

Broan MP140

141

85W

.08

Broan MP200

200

140W

.07

DEC Quiet Vent

165-295

120W

.06-.12

Fantech CVS 275

230

115W

.09

Notes:

(1) The flow, input wattage, and sound ratings are manufacturers' data provided to the author.

(2) Home Energy calculated the fan efficiencies from the input wattage and the flow at the two static pressure points provided.

(3) There are no sound ratings currently available for remote single-point or multipoint fans, but these fans are generally relatively quiet in application because the fan motor is located away from the living space.

Dedicated Fan
A centrally located, quiet ceiling fan acts as the general IAQ ventilation fan. Often it is located in the hall or at the top of the stairs. All spot ventilation is handled by other fans. A 24-hour clock timer is wired to the fan so that occupants can schedule ventilation times.

The dedicated fan should be quiet enough not to disturb occupants, especially if it is located near bedrooms. Generally a sound rating of 1.5 sones will be acceptable, but the quieter the fan, the less likely it is that residents will complain. Fans rated at 0.5, 0.8, and 1.0 sones should be considered, since people aren't used to having a fan run for extended periods.

Pictured here is the remote (main) unit for a multi-point exhaust fan system. With pick-ups in several locations, this single-motor system provides quiet ventilation throughout the house.

Ducted Central Exhaust Fan
A remote-mounted central exhaust fan serves as the general IAQ fan; often it is installed in the attic or basement. It may be an inline fan with a single pickup or a multipoint fan with 3 to 6 pickups (see Bathroom Exhaust Fans, HE Nov/Dec '95, p. 29). It may provide spot ventilation from one or more locations in addition to providing the general ventilation. Remote-mounted fans have the advantage of reducing noise in the house. With no ceiling-mounted fans, all the occupant hears is air noise at the grille. Bathrooms, laundry rooms, and spa rooms that are not connected to the central system have conventional spot ventilation fans, but generally the central fan is sized to pick up all the ventilation locations. Two control strategies may be used:

The central fan operates continuously at a low rate to meet both the spot ventilation rate and the general IAQ rate. In this case, no controls are needed.

This remote single-point fan can be used for quiet bathroom exhaust, make-up air supply, or clothes dryer or range hood boosting.

Heat Recovery Ventilation
Heat recovered from stale exhaust air can be used either to heat incoming ventilation air or to make hot water. A heat recovery ventilator typically recovers 60%-75% of the waste heat in the exhaust air from the house and sometimes from the spot ventilation fans.

Although most places in the United States are not cold enough or do not have utility rates high enough to make them cost-effective, heat exchangers have other advantages. They filter fresh air coming in and temper it for comfort, and they better balance the flow of incoming fresh air and exhausted stale air.

A traditional heat recovery ventilator (HRV) or energy recovery ventilator (ERV) generally uses a fixed plastic or aluminum core or a rotating plastic wheel to move heat from the stale exhaust air to the incoming fresh air. An HRV transfers mostly sensible heat, while an ERV can recover additional heat, due to the latent heat of moisture transfer. The colder the climate, the more energy is saved by warming the incoming ventilation air. The HRV can also be used in warm climates to precool outdoor air, to reduce the air conditioning load. However, the designer should make sure that the heat exchanger has the ability to accommodate moist air without causing condensation problems. Exhaust ducts run from pickup points in bathrooms and in the living space to the HRV unit. Supply ducts to the main living space and the bedrooms or to the furnace ductwork deliver the tempered outdoor air to the occupied spaces. Supply and exhaust ducts are run from the HRV unit to the outside. Generally two fans operate within the HRV cabinet to bring in outdoor air and exhaust stale indoor air. Look at and compare the efficiency ratings by the Home Ventilating Institute (HVI) when considering HRV equipment.

An HRV unit is usually either controlled by a 24-hour timer or operated continuously. Some units have two or more ventilation speeds, and most offer defrost capability to keep surfaces from frosting up in cold weather. A ducted range hood provides spot ventilation in the kitchen, since grease from cooking should not be pulled into the HRV ducts.

Avoiding Backdrafting

Care should be taken to avoid backdrafting combustion appliances with negative pressure from ventilation fans. While the general IAQ system should be run for long periods, higher-CFM fans, such as range hoods, downdraft range exhausts, and dryers, will often cause larger problems with carbon monoxide (CO) and other contaminants. Gas water heater pilot lights and even supposedly airtight wood stoves with outside-combustion air can be backdrafted at only 2-4 Pa of negative pressure. The best strategy is to use only sealed combustion appliances and to avoid wood stoves and fireplaces. If the presence of combustion appliances makes backdrafting a concern, avoid large spot exhaust fans, use balanced-flow ventilation with HRVs or blending ventilators, and install a CO detector. Houses will virtually always go negative under certain conditions, even if a balanced-flow system is installed.

A ventilating heat pump water heater (VHPWH) uses a small dedicated heat pump to recover heat from ventilation exhaust air and transfer it to a hot water tank. The fan on the VHPWH unit acts as the general ventilation fan; ventilating eight hours a day, the VHPWH can generally make all the hot water needed for a typical family of four from waste heat in the exhaust air. This unit is different from a heat pump water heater sitting in a room, since the fan must be capable of pulling stale air from several rooms and of exhausting the stale air outside. In a cooling climate, the air flow for some units can be reversed in the summer to cool and dehumidify incoming outdoor air as a supply ventilator. Exhaust ductwork is run from one or more pickups in the house to provide general IAQ ventilation and possibly some spot ventilation. Outdoor air is introduced into bedrooms and living spaces through vented windows or wall vents. A ducted range hood provides spot ventilation in the kitchen. Bathrooms, laundry rooms, and spas that are not connected to the VHPWH system have conventional spot ventilation fans. At least two companies offer VHPWH units in the United States-DEC Therma-Stor and E-Tech.

A dehumidifying ventilator uses a small compressor to dehumidify and reheat the indoor air while introducing outdoor air to blend with it. This supply-side ventilator can be operated either as a positive-pressure system by itself in warm climates or as a balanced-flow system in cold climates if a quiet, low-energy fan is used to exhaust a like amount of indoor air. It includes a high-efficiency air filter to clean the blended air to near-HEPA quality. (HEPA stands for High Efficiency Particulate Arrestor. It is a filter capable of filtering out 99% of particles 1 micron or larger. The filter on the dehumidifying ventilator removes 95% of particles 1 micron or larger, so it is called near HEPA.) This type of ventilation can be used in areas of medium to high temperatures and high humidity to dehumidify and clean the indoor air efficiently, while introducing outdoor air. When it is relatively cool but humid, as at night, it will dehumidify the air and improve comfort even when the air conditioner does not get a call for cooling. Dehumidifying ventilators are available from American ALDES and DEC Therma-Stor.

Compact Fluorescent Fan/Light Coming Soon

In the spring of 1996, a compact fluorescent light option will be available on Panasonic's 70, 90, and 110 CFM quiet ventilation fans. This new fan/light combination will use a separate ballast and two 13W lamps to provide a fan and light with a combined draw as low as 41W, rather than the 150W-200W of a conventional fan/light. It is designed for both new construction and retrofit.

Meeting Outdoor Air Requirements
The amount of outdoor air being brought into the house should equal the general IAQ ventilation rate discussed earlier (0.35-0.50 ACH or 5 CFM per 100 ft2, but not less than 15 CFM per person). Outdoor air can be supplied in several ways.

Wall or window vents must allow air to flow at low pressures and should be tested according to the HVI test standard. (The HVI is the same organization that certifies the air flow of fans and the efficiency of HRVs.) Field testing shows that only a small portion of the outdoor air that comes into the structure actually passes through these vents. The use of the vents, however, ensures that at least some of the outdoor air is introduced into the spaces that most need fresh air, such as bedrooms.

One way of supplying outdoor air to a house, wall vents should be positioned high on walls and away from seating areas and beds, since they may produce drafts.

Wall vents are generally made of plastic and/or metal and include a rain cap for the outside, an insect screen, and a cleanable filter. They also include a flow control mechanism to limit or stop air flow, a liner to pass through the wall cavity, and a device to direct the airstream away from people in the room. Wall vents should be located high on the wall and away from seating areas and beds, to minimize complaints about drafts. European products manufactured for this purpose have been used throughout the United States for several years. They are available from American ALDES (Airlet models 100 and 200) and DEC Therma-Stor (FRESH 80 and FRESH 100).

Window vents are generally located in the upper sash area of the window. They usually have a rain shield and an insect screen, but no filter. Occupants can control the air flow by opening and closing the vent. The design may be as simple as drilled holes in the vinyl frame with a sliding shutter. Window vents vary in size, but they all must be tested in accordance with the HVI procedure. Depending on the manufacturer, the upper sight line of the window may be affected, since the vent can make the frame taller. Many vinyl and aluminum and some wood window manufacturers now offer vented windows. Window vents are manufactured by Titon Incorporated and American ALDES.

Instead of wall or window vents, some systems use an outdoor air connection to a furnace return air plenum. The connection should be between 6 and 8 inches in diameter to reduce pressure drop, depending on the flow required. In the Pacific Northwest, we require that the duct be a minimum of 6 inches for up to 80 CFM, 7 inches for 80-120 CFM, and 8 inches for 120 CFM or more, assuming up to 20 ft of run from the outside. For best operation, a motorized damper should be used to limit the intake of outdoor air to periods when the exhaust fan is being operated for general IAQ ventilation. Otherwise the furnace will pressurize the house if the IAQ fan is off. The furnace air handler must also be turned on for circulation and to draw in the outdoor air. Honeywell and DuroZone both make controllers that turn on the air handler and the exhaust fan and open the motorized damper.

Although connecting to the furnace is relatively simple, using a 400W air handler fan motor to pull in outdoor air and circulate it wastes a lot of energy compared to a low-energy fan, such as Panasonic's 15W 70 CFM model. The furnace fan strategy would increase electrical consumption by over 1,000 kWh per year. (Using a variable-speed furnace fan would reduce the fan motor's energy use, but the pressure in the return air plenum-necessary to bring in fresh air-would be reduced even more, making it ineffective.)

Window vents allow occupants to control air flow by opening and closing the vent located in the upper sash area of the window. While separate vent units have been most common in the past, some manufacturers now offer vented windows.

Sizing Fans
Most bath fans are designed to provide at least 50 CFM. Whenever ducts, grilles, wall caps, or roof caps are connected to a fan, the fan has to work harder to move air. Fan flow ratings are based on a particular amount of resistance to air flow, described as static pressure. Most bath fans are rated at 0.1 inches of water gauge (also known as water column), often abbreviated as 0.1 wg. This is roughly the pressure drop created by 50 CFM of flow through a grille, 5 ft of 3-inch flex duct, and a wall cap for the fan. Bath and range hood fans have been tested and certified by the HVI at flow ratings of 0.1 wg and sometimes at 0.25 wg. The product literature for many fans includes a table with the fan flow rates at a variety of static pressures.

Most actual duct installations are longer than 5 ft and/or have elbows, which makes the resistance greater than the HVI test assumes. To compensate for duct resistance, many utility programs and building codes require that the fan's air flow be rated at a higher static pressure than 0.1 wg, or that the fan be increased in size (flow rate). For example, the State of Washington's ventilation code requires that the fan's flow be certified at 0.25 wg. It's best to plan on using the flow at 0.25 wg to meet required flow, since this is more likely to be the typical static pressure when fans are installed.

Ducts and
Terminal Devices
Ducts need to be sized correctly to deliver the desired air-flow rate. Flexible duct has about twice as much resistance to air flow as smooth metal duct, but either type can usually be used for bath fans. When using flex duct, keep runs short. Avoid droops that increase resistance and create a place for moisture to condense. Avoid using plastic dryer duct connectors. Use at least 4-inch ducts for fans of less than 80 CFM and 5-inch ducts for fans up to 120 CFM. Flex duct is acceptable, but smooth duct provides better air flow. For kitchen range hoods, only smooth metal duct meets the Uniform Mechanical Code and other model codes. All duct joints, fittings, and adjustable elbows need to be sealed with duct mastic or high-quality duct tape.

All fan ducts need to terminate outside the building shell-not in an attic or a soffit, or under the house. With a solid duct connection, moisture is carried outside the building shell so that it cannot condense inside a wall or on the bottom of the roof. Exhaust ducts should connect directly to a collar that extends through the wall or ceiling to a termination designed for exhaust air flow.

ASHRAE Standard Revisions

The 1996 revision to the ASHRAE ventilation standard currently under review will define and require ventilation systems to be capable of providing continuous ventilation, which I have labeled general IAQ ventilation in this article. This type of ventilation is also known as whole house ventilation in the Pacific Northwest. The draft revision includes three methods of providing continuous ventilation. While ASHRAE does not define the sound level for continuous-ventilation fans, it does require that fan sound be addressed.

HVI plans to define fans for continuous ventilation as fans with a sound rating of 1.5 sones or less if ceiling mounted. HVI is developing a sound testing standard for remote-mounted fans. It is advisable to use sound-absorbing ducting with an HRV, a blending ventilator, or a remote-mounted fan to ensure quiet operation. (A blending ventilator tempers outside air with house air to make the air flow somewhat more comfortable without using a heat exchanger.) The continuous ventilation fan will be required to have a certified flow rating at 0.25 wg for ceiling-mounted fans and 0.4 wg for remote-mounted fans and HRVs.

Timers and
Other Controls
For effective ventilation, the general ventilation fan needs to operate when contaminants are present or are being produced. The ASHRAE standard recommends ventilation whenever the house is occupied. Generally, a good compromise in practice is to ventilate 8 to 12 hours a day. Ventilation can be scheduled by using a 24-hour timer set for at least two periods. Part of the ventilation time should be scheduled during the night, when people are sleeping. For instance, a fan could be scheduled for 6 pm-12 midnight and 6 am-10 am for a household with nobody home during the day, while a parent home with preschool children might want to ventilate 24 hours a day.

Controls are now available that perform several functions. Honeywell has an integrated comfort setter that combines a thermostat with a ventilation control, a humidity sensor, and an outdoor thermometer. Called the PC8900, it allows the installer to program the ventilation run time and remembers if the air handler ran in the last half hour. American ALDES and Tamarack Technologies offer a combination timer and speed control called the Airetrak. This device allows the use of a larger fan that can be set to run at a lower and quieter speed for general ventilation; it includes a push-button override to full speed for 20 minutes, and a programmable duty cycle for general ventilation. The Airetrack was developed to be used with the new condenser motor in a Panasonic fan; it is the only speed control that Panasonic allows to be used with its fans.

These two-unit condominiums are part of the Winslow Cohousing Project on Bainbridge Island, WA. The units use ventilating heat-pump water heaters (also called heat-recovery water heaters) to provide both ventilation and hot water.

If the ventilation system is designed to operate continuously to provide both spot ventilation and general ventilation, no timer is needed. When the general ventilation fan is also used as an intermittent spot fan, a switch or interval timer must be installed so that people can turn the fan on when needed.

Additional ventilation time may be needed during large parties or when someone is working on hobbies or other activities that produce noxious odors or extra moisture.

Go Forth and Ventilate
The old adage to build it tight and ventilate it right still holds true. The designer, the builder, and the occupant are all best served by building the home relatively tight and then installing and operating a mechanical ventilation system that is appropriate for the house, the occupants' lifestyle, and the climate.
Resources
The Certified Home Ventilating Products Directory, The Home Ventilating Institute (HVI), 30 West University Dr., Arlington Heights, IL 60004-1893. Tel:(708)394-0150.